Despite many years of promising new therapies, cancer remains a major cause of morbidity and mortality (Bailar et al., N. Engl. J. Med. 336:1569–1574, 1997). Typical therapeutic treatments may include surgery, radiation therapy, and/or chemotherapy. A variety of chemotherapeutic compounds have found widespread use in the clinical treatment of cancer and have proven to be indispensable in the effort to combat this disease. Nevertheless, on an individual case basis, some cancers can respond differently to a given therapy making it difficult and costly to achieve an effective clinical treatment regimen. Accordingly, there is a substantial need for new methods and active agents that are effective in inhibiting the growth of tumors.